Recovery of hydrocarbons by in-situ hydrogenation



Sept. 28, 1965 J. N. DEW ETAL 3,203,514

RECOVERY OF HYDROCARBONS BY IN-SITU HYDROGENATION Filed Oct. 51, 1962 2Sheets-Sheet 1 k 35 HEA 7'50 C004 (At/602N647 Z ONE- ZOA/ S &

POE/770M M/ FOE/14.4 770 INVENTORS ATTOPNE-Y Sept. 28, 1965 J. N. DEWETAL 3,203,514

RECOVERY OF HYDROCARBONS BY INSITU HYDROGENATION Filed Oct. 31. 1962 2Sheets-Sheet 2 OIL RES/DUAL AFTER PEI 5'26? COMBUSTION,

HYDROGEN EACTED M/ 2/ MIA/U755, 56'F/5BL F I INVENTORS E Z Max-1N M 02w,4

WILL/AM L. MAW/7v United States Patent 3,208,514 RECOVERY OFHYDROCARBONS BY IN-SITU HYDROGENATION John N. Dew and William L. Martin,Ponca City, Okla,

assignors to Continental Oil Company, Ponca City,

Okla, a corporation of Delaware Filed Oct. 31, 1962, Ser. No. 234,383 17Claims. (Cl. 166-2) This invention relates to secondary recovery methodsfor producing subterranean deposits of crude oil. More particularly, thepresent invention relates to a method for hydrogenating undergrounddeposits of crude oil, thereby upgrading the oil and lowering itsviscosity so that it may be more easily moved through a formation to aproducing well.

As is well known in the art after recovery from a subterraneanformation, crude petroleum requires removal of certain materials, suchas sulfur, oxygen and nitrogen in order to prevent interference by thesematerials with subsequent refining operations. The removal of thesematerials has been accomplished previously by hydrorefining proceduresin which the recovered crude oil is subjected to contact with hydrogenin suitable reaction vessels under requisite conditions of temperatureand pressure prior to further refining operations. During such contact,reduction reactions occur in which the loosely bound oxygen, nitrogenand sulfur in the crude oil are converted to more easily removed water,ammonia and hydrogen sulfide, respectively. Further, by using excesshydrogen, high pressures, and appropriate temperature ranges, desirabledestructive hydrogenation of the crude oil may be made to occur wherebystable, lower molecular weight materials are formed which are moresuitable stocks for use in the production of gasoline. Destructivehydrogenation also results in the reduction of the viscosity of thecrude oil.

It has heretofore been recognized that if there were available somepractical method for hydrogenating petroleum deposits in placethat is,in their natural subterranean locationsa very considerable economicadvantage would be achieved over the present hydro-refining proceduresconducted at the surface in that the necessity for reaction vessels andother equipment might be obviated. Moreover, the consequent lowering ofthe viscosity of the crude oil by in-situ hydrogenation would makepossible the recovery of large quantities of low gravity, viscous crudeoils which have previously not been possible to recover by conventionalrecovery methods.

A relatively recently proposed process for the in-situ hydrogenation ofviscous crude oils suggests that the hydrogenation reaction may becarried out by placing electrical heating units in casings extendingradially into the formation from a bore hole, passing ahydrogen-containing gas through the casings and over the heating units,and finally, contacting the viscous crude oil with the heated hydrogento hydrogenate the oil. The hydrogenated oil is reduced in viscosity andflows back into the bore hole through the radially extending casings.The described method of hydrogenation poses a number of problems and hasenjoyed very little commercial success. Only small parts of anoil-bearing formation can actually be heated to the temperaturenecessary to achieve substantial hydrogenation. Moreover, the electricalheaters and the equipment necessary to position the radially extendingcasings are expensive and render use of the process economicallyunfeasible in many instances.

The present invention provides an improved, more practical method forrecovering crude oil from subterranean formations by the process ofin-situ hydrogenation,

Patented Sept. 28, 1965 and achieves relatively improved results byreason of the establishment in the formation of an environment andconditions which are conducive to eflicient hydrogenation of thehydrocarbon materials therein. The method of this invention initiallyprovides for the conversion of the crude oil in the major part of theformation to a heated, hydrogenatable carbonaceous material. Thehydrogenatable carbonaceous material is, by this initial step, heated toa temperature which is optium for the progression of the hydrogenationreaction.

A hydrogen-containing gas is next injected into the formation in asufiicient amount and at a sufficient pressure to assure the efiicientprogression of the hydrogenation reaction and an adequate supply ofhydrogen is utilized to assure the substantially complete hydrogenationof the carbonaceous residue.

As a final step of the method, the hydrogenated material of loweredviscosity is produced from the forma tion by passing a fire or waterflood therethrough in accordance with these conventional and well knownsecondary recovery techniques.

To yet more specifically summarize the method of the present invention,the crude oil in the formation is initially converted to soft coke andoil-like, hydrogenatable material by passing a combustion front throughthe formation. A preferred technique for propagating the combustionfront through the formation is that which is termed reverse in-situcombustion, the details of which are discussed hereinafter. Forwarddrive in-situ combustion, as contrasted with reverse in-situ combustion,may also be utilized to convert the oil to hydrogenatable material andto heat the material sufficiently for the subsequent progression of thehydrogenation reaction. However, for reasons subsequently explainedherein, the maintenance of the temperature of the combustion frontwithin optimum limits for the production of hydrogenatable carbonaceousmaterial is more difficult in the case of forward drive in-situcombustion than in the reverse in-situ combustion technique. Whenutilizing reverse in situ combustion, the temperature at the combustionfront is maintained in the range of from about 400 F. to about 1000 F.,and preferably from about 400 F. to about 850 F., so that the residualhydrocarbon material left in the formation after passage of thecombustion front therethrough is maximized in quantity and is heated toa temperature within the optimum range for the progression of thesubsequent reaction by which the material is hydrogenated.

After the passage of the combustion front through the formation, ahydrogen-containing gas is injected into the formation and is maintainedtherein at a pressure of at least 500 p.s.i.g. and a temperature of fromabout 400 F. to about 950 F. for a period of time suflicient tohydrogenate the residual carbonaceous material remaining in theformation. This period of time varies considerably, according to thetemperature and pressure employed and the nature of the formation. Asindicated above, the completion of the hydrogenation reaction within theformation is followed by the recovery of the hydrogenated material oflowered viscosity by any suitable technique, such as fire flooding orwater flooding.

From the foregoing description of the present invention, it will havebecome apparent that an important object of the invention is to provide.an improved method of hydrogenating viscous petroleum deposits in placeso that such deposits may be more easily and more completely recoveredfrom the formation in which they occur.

Another object of the present invention is to provide a process for thein-situ hydrogenation of viscous petroleum deposits, which process maybe more economically practiced than those which have heretofore beenproposed.

A further object of the present invention is to provide a method foreffecting the recovery of crude oil from a subterranean formation in anupgraded, partially refined condition. Other objects and advantages ofthe invention will be apparent upon reading the following detaileddescription of the invention in conjunction with the accompanyingdrawings.

In the drawings, FIGURE 1 schematically illustrates the manner in whichone embodiment of the invention is practiced. FIGURE 2 is a graphshowing the relation between the combustion front temperature and theamount of residual material left in the formation, and also showing therate of progression of the hydrogenation reaction as the temperature ofthe residual material to be hydrogenated is varied.

In hydrogenating crude oil, the oil must be subjected to a minimumtemperature of 400 F. in order for the hydrogenation reaction to proceedefficiently. Although the reaction is exothermic, it does not developsuflicient heat to sustain the reaction throughout the formation and tohydrogenate the oil to the maximum extent. Thus, even assuming that allof the oil in the formation were available for hydrogenation and thatthe maximum possible amount of hydrogen (about 460 standard cubic feetper barrel) were reacted therewith, the formation temperature would onlybe raised about 100 F. by the exothermic heat of reaction. From theseconsiderations, it will be apparent that supplementary heat must beprovided to raise the temperature of the formation to above 400 F.

One of the widely used techniques of secondary recovery of deposits ofless viscous petroleum is that of in-situ combustion. Because of theemployment in this procedure of a high temperature combustion frontwhich is propagated through the formation, we have determined that thistechnique offers a suitable and relatively economical supplementarysource of heat for raising the temperature in the formation to a levelsufficiently high to support a subsequent hydrogenation reaction.

Two general types of in-situ combustion are presently practiced. In theso-called forward drive in-situ combustion, the fire or combustion frontis initiated in the formation adjacent the injection well and propagatedthrough the formation from an injection Well toward a producing well orwells. A driving, combustion-supporting gas is introduced into theformation from the injection well and moves to, and across, thecombustion front toward the producing wells. The combustion-supportinggas (oxygen-containing gas) promotes the combustion of a portion of thehydrocarbon at the combustion front and drives the hydrocarbon gases.and less viscous hydrocarbons ahead of the front toward the producingwells where they are ultimately recovered.

The second type of in-situ combustion secondary recovery is referred toas reverse or inverse in-situ combustion. In this procedure, thecombustion front is initiated in the formation adjacent the producingwell or Wells and moves through the formation toward an injection wellspaced from the producing wells. The combustion-supporting gas, on theother hand, is injected into the formation through the injection welland thus moves through the formation in the opposite direction to thedirection in which the combustion front is propagated. The hydrocarbonswhich are highly heated by the passage of the combustion front, butwhich are not combusted, are driven by the combustion-supporting gasthrough the formation from the combustion front to the producing wells.

In both forward drive and reverse in-situ combustion, varying amounts ofcoke, tarry material and heavy viscous petroleum remain in the formationafter the passage of the combustion front. Some or substantially all ofthis material is hydrogenatable, depending upon the temperature of thecombustion front being passed through the formation, whether a forwarddrive or reverse in-situ combustion technique is used, and theconditions of temperature and pressure imposed during the subjection ofthe residual material to contact with hydrogen or a hydrogencontaininggas during the hydogenation.

When a forward drive is utilized to propagate a combustion front throughthe formation, we have found that when the temperature at the combustionfront exceeds 700 F., very little residual carbonaceous material is leftin the formation after passage of the front therethrough. Moreover, inthe forward drive in-situ combustion, when the front temperature exceeds700 F., the carbonaceous material which does remain in the formation islargely constituted by hard coke which is very difiicult to hydrogenate.At temperatures not exceeding 600 F., however, a substantial deposit ofresidual hydrogenatable carbonaceous material is left in the formationafter passage of the front. It is frequently diflicult to conduct theforward drive in-situ hydrogenation with the temperature of thecombustion front maintained as low, or lower, than 700 P. so as to leaveany appreciable quantity of hydrogenatable carbonaceous material in theformation. However, in some instances, the flux rate at which thecombustion-supporting gas is injected may be reduced substantiallywithout falling to sustain the drive, thereby lowering the temperatureat the combustion front, and yielding a larger residual amount of moreeasily hydrogenated soft coke and tarry materials. In suchcircumstances, subsequent recovery of the residual material by in-situhydrogenation, as hereinafter described, becomes economically feasibleand may be coupled with the preheating afforded by the forward drivein-situ combustion to upgrade and recover the tarry and oily materialsremaining in the pores of the formation. This allows recovery of theresidual material normally coked or combusted, thereby increasingrecoveries.

As hereinbefore indicated, however, the reverse in-situ combustionprocedure is preferred as a preheating or supplementary heatingtechnique preceding the in-situ hydrogenation under almost all formationconditions, including that described above wherein the forward drivecombustion is effective for preheating. This is due to the fact that thetemperature at the combustion front is more easily controlled in reversein-situ combustion, lower flux rates of combustion-supporting gas arerequired and a substantially larger portion of the crude oil originallypresent is retained in the interstices of the formation ashydrogenatable residual material when operating at temperatures between600 and 1000 F. Moreover, a much larger portion of the zone which hasbeen swept out by the moving combustion front may be retained at atemperature within the optimum range for the progression of thehydrogenation reaction. This is possible by reason of the continuouspassage of heated gases and hydrocarbons en route to the productionwells through this swept out zone which has already been heated bypassage therethrough of the combustion front as it is propagated towardthe injection well.

The manner in which residual hydrogenatable carbonaceous material isdeposited and retained in a heated condition in the formation by thepropagation of a combustion front therethrough by the preferred reversedrive technique will be more clearly understood when reference is madeto FIGURE 1 of the drawings.

In FIGURE 1, reference character 10 refers to a subterranean formationin which is located a deposit of crude oil which it is proposed toupgrade and recover by the in-situ hydrogenation process of the presentinvention. The subterranean formation 10 is traversed by a pair ofspaced wells 12 and 14 which may be employed as the injection well andthe producing well, respectively. It. will be understood, of course,that a plurality of injection wells and producing wells may also beutilized in a manner similar to that disclosed in United States Patent2,958,519 to J. R. Hurley. In contradistinction to the forward driveprocedure of originating a combustion front at or adjacent the injectionwell 12 and propagating the.

front toward the production well 14, in the reverse in-situ combustiontechnique, the hydrocarbons in the formation are ignited adjacent theproduction well 14 and the combustion front designated by referencenumeral 16 moves away from the production well 14 and toward theinjection Well 12.

Combustion may be initiated by any suitable means, such as an air-gasburner or electrical heater. While the formation hydrocarbons are beingignited, a combustionsupportinggas, such as air or oxygen, is passedinto the formation through the production well 14 for a short period oftime so as to establish and enlarge the combustion front and propagatethis front a short distance outwardly in the formation away from thebase of the production well 14.

Injection of combustion-supporting gas through the production well 14 isthen terminated and the injection well 12 is then utilized to supply airor other combustionsupporting gas to the formation. As air is passedinto the formation from the injection well 12, the -air moves throughthe formation and crosses the combustion front 16 where oxygen reactswith the hydrocarbons. The gaseous combustion products pass on through ahigh temperature zone 18 which has been swept out by the front 16 untilthey reach the production well 14. The air moving through the formationfrom the injection well 12 toward the production well 14 supportscombustion at the combustion front 16 and serves to drive hot oilvapors, gaseous cracked products and low viscosity liquid hydrocarbonsfrom the combustion front 16 through the heated zone 18 to theproduction well 14.

As the combustion front 16 moves through the formation, it enlarges theheated area 18 and leaves in the interstices of the formation withinthis area a deposit of soft coke, tarry materials and viscous liquidhydrocarbons. The zone 18 remains very hot as a result of the continuedflow of heated hydrocarbon gases and liquids therethrough from thecombustion front 16 to the production well 14.

The relative temperatures in the unburned portion of the formation aheadof the moving combustion front .16 and the heated portion 18 of theformation are illustrated bythe temperature profile graphicallyportrayed in the lower part of FIGURE 1. It will there be noted that theheated portion 18 of the formation 10 and the residual carbonaceousmaterial which remains therein after the passage of the combustion front16 therethrough are retained at a temperature which is only slightlylower than the temperature existing at the combustion front 16. Thus, ifthe temperature in the combustion front 16 can be brought within therange which is suitable for the subsequent in-situ hydrogenation of theresidual material remaining in the formation, the heated portion 18 ofthe formation 10 which remains after passage of the front therethroughmay also be retained in substantially the same range.

We have determined that by controlling the flux rate of air introducedto the formation 10 via the injection well 12, the temperature of thecombustion front 16 may be kept within a range which is most suitablefor the deposition of a maximum amount of residual hydrogenatablematerial and for the progression of the hydrogenation reaction. Since,as indicated above, the hydrogenation reaction cannot be carried outsuccessfully at temperatures lower than 400 R, such temperatureconstitutes the lowest temperature at which the combustion front 16should be maintained.

With respect to the upper limit of temperature which should optimallyobtain at the combustion front 16, laboratory tests have shown that,though some hydrogenatable residual material remains in the formationwhen a reverse combustion front having a temperature as high as 1000 F.is passed therethrough, temperatures not exceeding 850 F. are preferredbecause operation at temperatures exceeding this value results inexcessive cracking of hydrocarbon materials in the formation andconsumes excessive fuel, with the result that more of the hydrocarbonmaterial is displaced from the formation and consumed by thecombustion-supporting gas passed therethrough. The relation between theamount of residual material left in the formation and the combustionfront temperature is portrayed by curve A in the graph illustrated inFIGURE 2.

Besides the disadvantage posed by the undesirably small quantities ofresidual material left in the formation when combustion fronttemperatures exceeding about 850 F. are utilized, the small amount ofmaterial which is left in the formation is primarily deposited as a hardcoke which is difficult to hydrogenate. At temperatures of about 850 F.or less, on the other hand, only an optimum degree of thermal reactionwill occur at the time of passage of the combustion front through theformation and during the subsequent hydrogenation. Also, as thetemperature in the combustion front 16 and in the heated zone 18 isdecreased from 850 F. toward the minimum operative temperature of 400F., increasing quantities of tarry and oil-like materials are left inthe pore spaces of the zone 18.

The subsequent hydrogenation react-ion is, in actuality, destructivehydrogenation in which the larger molecules of the hydrocarbon materialare cracked and the products of the cracking are subsequentlyhydrogenated to reduce their viscosity and permit them to be more easilyrecovered from the formation. Temperatures exceeding about 500 F. arerequired for any substantial degree of cracking to occur and attemperatures exceeding about 850 F., the cracking reaction becomesundesirably predominant relative to the destructive hydrogenationreaction in the sense that more solid coke .is produced as a result ofsuch cracking and less hydrogenated liquid material is produced as aresult of destructive hydrogenation. Carrying out the destructivehydrogenation at superatmospheric pressures of at least 500 p.s.i.g. inthe manner hereinafter prescribed greatly aids in suppressing cokeformation as a result of cracking and in promoting the formation of lowviscosity, hydrogenated liquid materials. In general, the higher .thepressure, the more the hydrogenation reaction is favored.

As will be noted from the graph shown in FIGURE 2, the hydrogenationreaction (as indicated by the consumption of hydrogen) proceeds mostrapidly at temperatures exceeding 600 F., and preferably falling withinthe range of from about 800 F. to 850 F. Thus, when considered only fromthe standpoint of achieving an optimum balance between the destructivehydrogenation and cracking reactions, a temperature of between about 800F. and 850 F. would appear to be indicated for most effectively carryingout the hydrogenation of the residual material in the formation.However, as has been indicated, the deposition of larger quantities ofhydrogenatable residual material is favored by the use of lowercombustion front temperatures, so that rather than operating in thetemperature range which would provide the maximum rate of progression ofthe hydrogenation reaction and still avoid excessive cracking and cokeformation, it is preferably to propagate a lower temperature combustionfront .through the formation to leave more hydrogenatable residualmaterial. Consequently, the hydrogenation reaction temperature is alsolowered from the otherwise optimum range of 800 F. to 850 P. so that, ineffect, a compromise temperature range of from about 600 F. to 700 F. ispreferably utilized for the hydrogenation reaction. Since thehydrogenation reaction per se generates approximately F. as theexothermic heat of reaction, this means that the combustion fronttemperature should be between about 500 F. and 600 F. for optimumoperation. At this temperature, curve A on the graph of FIGURE 2indicate that between about 55 percent and 70 percent of the total oilin the formation remains as residual hydrogenatable material.

Although the hydrogenation reaction rate is considerably slower in thepreferred temperature range of from 600 F. to 700 F. than at highertemperatures, it may be increased by hydrogenating at higher pressures.Moreover, in in-situ hydrogenation of subterranean oil deposits, thereaction rate is not especially critical, since the hydrogen gas may beinjected into the formation, the injection and production wells thenshut in, and the reaction allowed to proceed to completion over severalweeks or months.

In order to maintain the combustion front 16 at a temperature such thatthe .temperature of the heated zone 18 of the formation 10 will fallwithin the broad operative range for the hydrogenation reaction of fromabout 400 F. to about 950 F., the air flux through the injection well 12should be adjusted to from about 2 standard cubic feet per hour persquare foot of the combustion front area to about 50 standard cubic feetper hour per square foot in the combustion front area. Thecombustion-supporting gas is continuously injected into the formationthrough the injection well 12 at a rate Within this range until thecombustion front 16 has reached the injection well.

In one embodiment of the invention, after the combustion front 16 hasreached the injection well 12, the injection of air or othercombustion-supporting gas into the formation is stopped. The injectionwell is then purged and the injection of hydrogen or ahydrogencontaining gas into the formation via the injection well 12 iscommenced. As an aid to maintaining the temperature in the burned-outformation within the range of from 400 F. to 850 R, which is desirable,or to within the range of from 600 F. to 700 R, which is most preferred,the hydrogenating gas may, if desired, be preheated to some temperatureabove about 400 F. prior to its injection into the formation. Thehydrogenating gas which is employed may be manufactured free hydrogen,or it may be a hydrogen-containing gas, such as the gaseous effluentfrom a catalytic reformer containing associated normally gaseoushydrocarbons, such as methane, ethane, propane, butane and the like.

The injection of the hydrogen or hydrogen-containing gas into theformation 10 is continued until free hydrogen is observed at theproduction well 14. At this time, the production well is closed in andthe injection of hydrogen is continued until the pressure in theformation exceeds 500 pounds p.s.i.g, and preferably is between about1000 pounds p.s.i.g and 4000 pounds p.s.i.g. We have found that, ingeneral, the higher the hydrogen pressure in the formation, the moreefficient the hydrogenation reaction which occurs therein and the fasterits rate of progression.

As discussed in detail hereinbefore, the rate of progression of thehydrogenation reaction has also been determined to be dependent, to asubstantial extent, upon the temperature obtaining within the formationduring the hydrogenation reaction. For example, based on laboratorytests, it was found that hydrogenation of the residual carbonaceousmaterial was completed in about five days when the formation temperaturewas about 700 F. and a pressure of 1000 pounds was maintained in theformation. When the temperature was lowered to 600 F. and thehydrogenation reaction was conducted at the same pressure of 1000p.s.i.g, thirty-five days were required to complete the hydrogenationreaction. When the pressure in the formation is increased to 4000p.s.i.g, the time required to complete the hydrogenation reaction isreduced by about one-third.

When the pressure within the formation has reached the desiredmagnitude, the injection well is then closed in and the entire formationremains pressurized and in a heated state for a sufficient length oftime for the hydrogenation reaction to reach equilibrium. As indicated,this period of time may vary over a wide range and will depend upon theformation temperature, the formation pressure, the amount of residualcarbonaceous material remaining in the formation, and the porosity andpermeability of the formation.

Following the completion of the hydrogenation reaction, the hydrogenatedmaterial in the formation is upgraded from the original material and isof substantially reduced viscosity. This lower viscosity material may berecovered from the formation by conventional water flooding, or aforward drive in-situ combustion recovery operation of the typehereinbefore described may be conducted to drive the hydrogenated oil tothe producing wells.

Several alternatives to the procedure hereinbefore described may bepracticed in carrying out the process of the invention without departurefrom the basic principles which are utilized. For example, after theformation has been traversed by the combustion front and the residualcarbonaceous material which remains therein is ready to be hydrogenated,the hydrogenating gas may be injected through the producing well,instead of the injection well, if this procedure is preferred. Also,economic advantages may frequently be gained by terminating the reversein-situ combustion prior to the propagation of the combustion front tothe injection well. In other words, the combustion front is moved only apart of the distance between the production well and the injection wellbefore the supply of combustion-supporting gas is terminated. Hydrogenor a hydrogen-containing gas is then injected into the formation at theproduction well and that portion of the formation which has been burnedout is the situs for the hydrogenation reaction. Following thehydrogenation reaction a forward drive in-situ combustion may beinitiated at either of the wells and the hydrogenated material, as wellas the material remaining in the portion of the formation which has notbeen traversed by the combustion front, recovered.

A further alternative procedure which is sometimes desirable is that ofpartially closing in the production well shortly prior to thetermination of the reverse insitu combustion so that the pressure in theformation commences to build up as a result of the continued injectionof the combustion-supporting gas through the injection well. Thepressure built up by virtue of the partially throttled production wellcontinues until a minimum pressure of about 500 pounds p.s.i.g. isachieved in the formation. At this point, the reverse in-situ combustionis terminated and a hydrogen-containing gas is then injected into theformation in the manner previously described to further increase thepressure therein to in excess of 1000 p.s.i.g. By this procedure, asaving in time is realized over the time required to build up thepressure in the formation to the required extent solely through theinjection of the hydrogenating gas. Moreover, the use of thecombustion-supporting gas to partially pressure the formation prior tothe injection of the hydrogenating gas results in an economic savingsince combustion-supporting gases, such as air or oxygen, are, ofcourse, much less expensive than hydrogen or hydrogen-containing gases.

While there have been shown, described and pointed out the fundamentalnovel features of this invention, as

'applied to the preferred embodiment entailing the use of a reversein-situ combustion in combination with insitu hydrogenation for therecovery of viscous crude oils from a subterranean formation, it will beunderstood that various omissions, substitutions and changes in the formand details of the process illustrated and described may be made bythose skilled in the art of petroleum production without departing fromthe spirit of the invention. It is therefore our intention to be limitedonly as required by the scope of the appended claims and reasonableequivalents thereof.

What is claimed is:

1. A secondary recovery method for recovering residual liquidhydrocarbons from a permeable underground formation traversed byinjection and production wells which comprises:

(a) continuously subjecting the hydrocarbons in the formation betweensaid wells to controlled in-situ combustion in which a combustion fronthaving a temperature of between about 400 F. and about 850 F. ispropagated through said formation to convert a portion of thehydrocarbons therein to hydrogenatable, residual carbonaceous material;

(b) contacting the residual carbonaceous material with a hydrogenatinggas at a pressure exceeding about 500 p.s.i.g. and a temperature of fromabout 400 F. to about 950 F. for a period sufiicient to substantiallylower the viscosity of said residual carbonaceous material by injectinghydrogenating gas through a first one of said wells;

(c) recovering said hydrogenated residual material from the formationthrough a second one of said wells.

2. A secondary recovery method as claimed in claim 1 wherein saidresidual carbonaceous material is contacted with a hydrogenating gas ata temperature of between about 600 F. and 700 F.

3. A secondary recovery method as claimed in claim 1 wherein saidcombustion front is propagated through said formation by passing acombustion-supporting gas through said formation from the second of saidwells to said first well at a rate of between 2 and 50 standard cubicfeet per hour per square foot of combustion front area.

4. A secondary recovery method as claimed in claim 3 wherein contact ofsaid residual hydrocarbons with a hydrogenating gas is effected by (a)partially shutting in said first well after the insitu combustion issubstantially completed to restrict the fiow of saidcombustion-supporting gas from said first well;

(b) continuing to pass combustion-supporting gas into said formation ata rate exceeding the rate at which said combustion-supporting gas flowsfrom said first well until said formation is back pressured to about 500p.s.i.g.; then (c) closing in said first well and terminating theintroduction of said combustion-supporting gas into the formation;

(d) introducing said hydrogenating gas into the formation to increasethe pressure therein to in excess of 1000 p.s.i.g.; and

(e) closing in said second well to maintain the pressure in saidformation in excess of 1000 p.s.i.g. while maintaining the temperaturetherein between 400 F. and 950 F.

5. The method claimed in claim 4 and further characterized to includethe steps of:

(a) blocking the escape of hydrogenating gas from the formation toincrease the pressure therein to in excess of 1000 p.s.i.g.;

(b) retaining the hydrogenating gas in the formation until the maximumhydrogenation of residual material has occurred.

6. The method claimed in claim 4 wherein the combustion-supporting fluidis introduced to said formation at a rate predetermined to maintain thetemperature at the combustion front at between about 500 F. and 600 F.

7. A method for hydrogenating subterranean hydrocarbons in place in aformation traversed by injection and production wells which comprises:

(a) continuously subjecting the hydrocarbons in said formation betweensaid wells to controlled reverse in-situ combustion in which a driving,combustionsupporting gas is passed through the formation toward aproducing well, said combustion front is 10 propagated in a directionopposite to the direction of movement of said combustion-supporting gas,and the rate of introduction into the formation of saidcombustion-supporting gas is controlled to maintain the temperature atthe combustion front between about 400 F. and about 850 F.;

(b) injecting a hydrogenating gas into said formation through one ofsaid wells to hydrogenate the residual carbonaceous material remainingin the formation following said controlled reverse in-situ combustion;and then (c) recovering said hydrogenated residual material from theformation through one of said wells.

8. The method claimed in claim 7 wherein said hydrogenating gas ishydrogen and the hydrogen is injected into the formation until apressure exceeding 1000 p.s.i.g. is reached in the formation.

9. The method claimed in claim 7 wherein said combustion-supporting gasis introduced to the formation at a rate of between 2 and 50 standardcubic feet per hour per square foot of formation area in the combustionfront.

10. A process for recovering residual carbonaceous materials remainingin a formation traversed by injection and production wells after passinga continuous reverse in-situ combustion front controlled to have atemperature in the range of about 400 F. to about 850 F. through theformation between said wells, which process comprises:

(a) injecting a hydrogenating gas into the formation through one of saidwells at a pressure exceeding 1000 p.s.i.g. while maintaining thetemperature in the formation between 400 F. and 850 F. so as tohydrogenate said residual carbonaceous materials; and

(b) recovering hydrogenated residual material from the formation throughone of said wells.

11. The process as claimed in claim 10 wherein said hydrogenating gas ishydrogen and the temperature of the carbonaceous materials in theformation during the injection of the hydrogen gas is maintained between500 F. and 700 F.

12. A process as claimed in claim 10 and further characterized toinclude the step of preheating the hydrogenating gas to at least 400 F.prior to the injection thereof into the formation.

13. A process as claimed in claim 10 wherein the hydrogenated residualmaterial is removed by water flooding the formation.

14. A process as claimed in claim 10 wherein the hydrogenated residualmaterial is recovered by passing a combustion front through theformation.

15. The method of recovering viscous crude oil from a subterraneanformation which comprises:

(a) penetrating the formation with a pair of wells spaced from eachother and adapted for use as an injection well and a producing well;

(b) initiating combustion of the oil in said formation adjacent theproducing well;

(c) continuously injecting a combustion-supporting gas into saidformation from said injection well to maintain the combustion of oil insaid formation and propagate the combustion front at a temperature ofbetween about 400 F. and about 850 F. in a direction away from saidproducing well and toward said injection well; then, following themovement of the combustion front from adjacent the producing well toadjacent the injection well;

((1) injecting a hydrogenating gas into the formation via one of saidwells while at least partially shutting in the other of said wells tobuild up the gas pressure in said formation to greater than 500 p.s.i.g.

(e) maintaining the hydrogenating gas in the formation at a pressureexceeding 500 p.s.i.g. until maxi- 11 mum hydrogenation of the residualcarbonaceous material in the formation is achieved; then (f) recoveringthe hydrogenated carbonaceous material from the formation through one ofsaid wells.

16. The method claimed in claim 15 wherein said hydrogenating gas ishydrogen.

17. The method claimed in claim 15 wherein the hydrogenating gaspressure in said formation is built up to in excess of 1000 p.s.i.g. andsaid pressure is maintained until maximum hydrogenation of the residualcarbonaceous material in the formation is achieved.

References Cited by the Examiner UNITED STATES PATENTS Pevere et a11661l Elkins 16611 Pevere et al. 166-1 Campion et al. 16611 Banks 166-11Fisher 166-11 10 BENJAMIN HERSH, Primary Examiner.

1. A SECONDARY RECOVERY METHOD FOR RECOVERING RESIDUAL LIQUIDHYDROCARBONS FROM A PERMEABLE UNDERGROUND FORMATION TRAVERSED BYINJECTION AND PRODUCTION WELLS WHICH COMPRISES: (A) CONTINUOUSLYSUBJECTING THE HYDROCARBONS IN THE FORMATION BETWEEN SAID WELLS TOCONTROLLED IN-SITU COMBUSTION IN WHICH A COMBUSTION FROM HAVING ATEMPERATURE OF BETWEEN ABOUT 400*F. AND ABOUT 850*F. IS PROPAGATEDTHROUGH SAID FORMATION TO CONVERT A PORTION OF THE HYDROCARBONS THEREINTO HYDROGENATABLE, RESIDUAL CARBONACEOUS MATERIAL; (B) CONTACTING THERESIDUAL CARBONACEOUS MATERIAL WITH A HYDROGENATING GAS AT A PRESSUREEXCEEDING ABOUT 500 P.S.I.G. AND A TEMPERATURE OF FROM ABOUT 400* F. TOABOUT 950*F. FOR A PERIOD SUFFICIENT TO SUBSTANTIALLY LOWER THEVISCOSITY OF SAID RESIDUAL CARBONACEOUS MATERIAL BY INJECTINGHYDROGENATING GAS THROUGH A FIRST ONE OF SAID WELLSE; (C) RECOVERINGSAID HYDROGENATED RESIDUAL MATERIAL FROM THE FORMATION THROUGH A SECONDONE OF SAID WELLS.